Directional communication using electronically steered antenna arrays will be a key ingredient in future wireless networks in the traditional sub-6 GHz bands and in the new millimeter-wave frequency bands. In first-generation mm-wave networks (e.g. IEEE 802.11ad wireless LAN), phased-array beamforming is used to obtain a directional and steerable antenna pattern. Due to its simplicity and power efficiency, the RF beamformer architecture shown in FIG. 1. View (A) has emerged as the preferred approach to perform the algorithmically simple spatial signal processing required in a phased array, namely to apply programmable complex-valued weights to the signals received at the elements of an antenna array. Application of the complex-valued weights can be implemented in the RF-domain through one of several approaches including phase-shifter/variable-gain amplifier combinations, or through vector modulators as shown in view (B). Another approach is to use the Cartesian-combining architecture shown in view (C).
Advanced multi-antenna-based spatial signal processing techniques are necessary to achieve higher spectral efficiency, network capacity and better interference management in future millimeter-wave networks. The digital beamformer architecture shown in FIG. 2 offers the highest flexibility in implementing such spatio-temporal signal processing. However, the high power consumption of the local oscillator (LO) distribution network, data converters and digital signal processing makes digital beamforming infeasible for a large number of antenna elements. Hybrid beamformers seek to strike a compromise by performing the bulk of the spatial processing for a large number of antennas at RF, along with a handful of downconversion chains to facilitate digital spatio-temporal processing. There are two types of hybrid beamformers—the “partially-connected or sub-array” type (i.e., PC-HBF) of FIG. 3 and the “fully-connected” type (i.e., FC-HBF) of FIG. 4. The fully-connected type can offer superior performance when compared to the partially-connected type at the expense of greater implementation complexity. The partially-connected type can be implemented using existing RF-domain phased arrays such as the one shown in FIG. 1 while implementations of the fully-connected type are not previously known in the literature.
It is anticipated that future millimeter-wave networks will be deployed in several widely separated bands. So far, the 28, 37, 39, 45, 57-71, 71-76, 81-86, and 94 GHz bands have been identified for commercial use. Deployment of standards will initially be in a few of the lower frequency bands, but the use of increasingly higher frequency bands will be necessary to address the anticipated demand for capacity and data rates. Another likely scenario is the adoption of different frequency bands in different regions. Therefore, we anticipate the need for reconfigurable, flexible beamformers that can operate in a reconfigurable manner in many widely separated frequency bands.